The invention relates to the field of plated plastic materials and, more particularly, to the metallic plating of certain plastic materials such that the plated article is solderable in a hot oil dip soldering process or a vapor phase soldering process.
Metal plated plastic materials are desirable in many applications due to their relatively low weight, relatively low cost and the relative ease and predictability of reproduction. Certain areas of the electronics industry are particularly suited for the application of metal plated plastic components. For example, a shipboard radar antenna which must be placed high in the ship's structure would be more desirable if its weight were minimized. Placing heavy antennas in high places on the ship will have the result of shifting upward the center of gravity position and will require offsetting amounts of weight placed below the water line. These additions of antenna weight and offsetting ballast weight have obvious disadvantages such as reduced speed of the ship, reduced range of the ship, degraded maneuvering capability, etc. The typical lower weight of plated plastic components many of which are of less weight than aluminum, used in a radar antenna application such as the above, could significantly lessen the overall ship weight gain. Likewise, reducing the weight of an aircraft radar system would have the obvious advantages of permitting flight at faster speeds, better maneuverability, range, etc.
Phased array antennas generally use a multitude of simple waveguide assemblies such as corporate feeds, sinuous feeds, multiport power dividers, diplexers, slotted radiators, etc. The costs of the manufacture and assembly of these components in the quantities used in many phased arrays can cause the overall cost of the antenna to be prohibitively high. Also, the weight of the antenna is substantial when these components are manufactured from metal materials.
Many of these assemblies are customarily manufactured from aluminum and are assembled using dip brazing techniques. In the dip brazing technique, a high temperature is used, i.e., above 538.degree. C. (1000.degree. F.), which typically distorts the aluminum part and requires subsequent steps of straightening, heat treating, cleaning and in some cases, machining the surfaces to obtain acceptable matching characteristics. Because of these requirements, repairs are generally not possible. The defective part typically must be cut away from the other parts and replaced if tolerances will permit. In addition, equipment costs for dip brazing are relatively high.
Using metal plated plastic electronic components in applications such as the waveguide assemblies described above would be of great value in the electronics area. Both the expense and weight of the assemblies would be reduced. However these metal plated plastic components ahould be compatible with aluminum and a process for interfacing them with aluminum would be required.
Plated plastic articles and processes for manufacturing them are well known in the art. For example, U.S. Pat. Nos. 4,148,945 and 4,078,096 both disclose processes for plating certain plastic materials. However, the products of these processes have thus far, not been suitable for the applications discussed above. As an example, if a waveguide feed and flange assembly is to be manufactured either partially or completely from plated plastics, certain characteristics of the plastic and its metallic plating would be required. The plated plastic must have the same thermal coefficient of expansion as the materials with which it will interface, compatible electrical properties and comparable strength characteristics. The plating must have good adhesion to the plastic material so that environmental stresses do not cause plating degradation with accompanying changes in electrical performance. The plated plastic components should be assemblable with each other and should be assemblable with metallic components in such a way that a solid joint is formed and electrical continuity is maintained where required. In some cases, such as where waveguides are to be formed of the plated plastic, it may be required that the joints be of high quality so as not to cause arcing within the waveguide.
The ability to solder plated plastics together or with aluminum would be very desirable. When used in a military application, plated plastic components must be consistent with interfacing metal components such as aluminum in order to satisfy military use standards. In regard to the above requirements, U.S. Pat. No. 4,148,945 discloses use of acrylonitrile/butadiene/styrene (commonly called ABS) as the plastic substrate material. This plastic typically has a low melting temperature, approximately 118.degree. C. (245.degree. F.), (see the Modern Plastics Encyclopedia, 1979, Vol. 56, No. 10A, pg 498), and has not been found to be suitable for soldering processes such as hot oil dip or vapor phase since it melts at the soldering temperature. U.S. Pat. No. 4,078,096 discloses the use of a polyimide polymer which, it is claimed, will withstand immersions (10-15 seconds) in a solder pot at 232.degree.-246.degree. C. (450.degree.-475.degree. F.). The melting temperature of this plastic is higher than ABS, however prior processes for metal plating have resulted in adhesion of the plating with peel strengths which are relatively low (less than 5 lbs./in.).
Many prior plated plastics are solderable using techniques such as wave soldering, soldering irons and other similar methods where the plastic substrate is not immersed in a relatively hot medium for an extended period. However in applications such as joining a waveguide flange to a waveguide section, an immersion soldering process, such as hot oil dip or vapor phase is very desirable since a fully flowed solder joint is obtained. However prior metal plated plastic materials have not been capable of withstanding a hot oil dip soldering process at temperatures around 243.degree. C. (470.degree. F.) for 60 seconds or more without experiencing either a deformation in or destruction of the substrate material or a degradation of the adhesion of the plating material to the substrate resulting in partial or complete delamination.
In view of the above, it would be a valuable contribution to the art to provide hot oil dip or vapor phase solderable, plated plastic components which are solderable to and compatible with like plated plastic components and with interfacing metal components such as aluminum.